Trees are essential workhorses in our environment, providing us with oxygen to breathe; foods such as apples, hazelnuts, and maple syrup to eat; and wood for our houses, paper products, and furniture. In nature, trees provide housing for birds, insects, and other animals. They filter water and absorb carbon. For more on all that trees do in our ecosystems, check out the article “Digging Deeper with Trees." Their importance is evident, but how do trees actually work?

Parts of a Tree

Trees are the world’s tallest freestanding organisms. They have an amazing design that allows them to soak up 1,400 liters of water every day (that’s like drinking enough water every day to fill 700 big soda bottles), and to do it against gravity. The basic parts of the tree are the roots, trunk, and crown.

Roots. Roots take up water and nutrients from the ground. They also anchor the tree in the soil. Most mature tree roots extend twice as far (widthwise) underground as the canopy of the leaves above the ground. Although some trees may also have a taproot extending deep into the soil, it is the roots closest to the surface that absorb the majority of the water.

Trunk. The trunk is the “highway,” or main distribution route, transporting water and nutrients from the roots and crown to the entire tree. The tree trunk and its main branches have five key parts, as illustrated by the Arbor Day Foundation.

Its outer bark protects the tree, holding moisture in and keeping excess moisture out, insulating it against temperature changes, and protecting it from insect damage. The outer bark is continually renewed from within the tree.

The inner bark, called the phloem, transports food to the rest of the tree, lives a short while, and then becomes cork and part of the outer bark.

The cambium cell layer is the part of the trunk that is actively growing. Every year, it makes new bark and new wood.

Sapwood, the next layer of the trunk, is new wood and is the transport system for moving water up to the leaves.

As the tree makes a newer ring of sapwood, the older sapwood becomes heartwood, the central layer that supports the tree. It is dead, but holds its strength as long as the outer layers are intact. It is super-strong, nearly as strong as steel.

Xylem tissue, which transports water, is found within the wood of the tree (both the sapwood and heartwood). The xylem vessels are tiny, each only several microns in diameter. When they die, their cell walls are still intact and continue to serve as a water transport pipeline.

Crown. The crown includes the large and small branches growing from the trunk, as well as their leaves. Crowns of deciduous trees hold many thousands of leaves. According to the Royal Forestry Society, a mature oak grows and sheds about 250,000 leaves a year.

Key Functions

Photosynthesis. To make food, trees need carbon dioxide, water, chlorophyll, and sunlight. Through photosynthesis, the leaves capture light energy from the sun and, in the presence of chlorophyll, convert it into sugars with carbon dioxide absorbed from the air and water taken up by the roots. The leaves release oxygen in the process.

Fluid Transportation. The network of xylem tissue found throughout the tree and phloem make up the tree’s vascular system. Water is absorbed from the soil by the roots. It moves to the water-transporting xylem vessels, a network connecting the roots, trunk, and canopy. Most of the water movement happens in the newer rings of growth. The phloem tissue, beneath the outer bark, carries the sugars made in the leaves downward and inward through the trunk, branches, and roots, or up into growing shoots.

Trees release water through leaf surfaces, or transpire, to keep from overheating under the hot summer sun. Transpiration also creates pressure to drive water up the tree. As water transpires from the leaves, negative pressure pulls more water from the xylem into the leaf. The tension continues to pull water through the branches, trunk, roots, and ultimately the soil.

Dormancy

To survive the winter’s stressful cold temperatures and wind gusts, trees rely on a key process called dormancy, a time when growth and development stop (except for the roots, which never really stop growing) and the tree conserves energy. Deciduous trees lose their leaves to conserve energy. Autumn’s short days and long nights trigger the tree to get ready. Evergreen trees keep their leaves throughout the year. They possess a thick layer of waxy covering on their leaves to help them conserve water year-round. The long, narrow shape of the leaves of pine trees and other conifers also helps reduce water loss.

During winter months, dormant trees may sustain some injuries such as frost cracks, when the pressure of ice is stronger than the wood, but trees that are otherwise healthy can heal from such injuries, similar to how we heal from a broken bone.

Following the prolonged cold, the new leaf buds that were produced the season before can start to grow, slowly at first, until temperatures warm. As soon as they start growing, the buds make hormones called auxins that stimulate growth in the rest of the tree, leading to the gorgeous green canopies we see in summertime!

For more information, check out:

Recommended Books

The Tremendous Tree Book by May Garelick and Barbara Brenner (ages 3-9) This delightful and informative book takes complex material about trees and pares it down to the basic facts that children can understand and enjoy.

The Giving Tree by Shel Silverstein (ages 4-8) This touching story illustrates the benefits trees provide to humans.

The Voice of the Wood by Claude Clement (ages 4-8) In this fable, a Venetian instrument maker loses a beloved old tree in his garden, but saves the wood to make a magical cello that will make music only if “the musician’s heart [is] in tune with the wood.”

Trees by Roland Ennos, Natural History Museum (for educators) In this richly illustrated overview, the author examines trees’ history, explains their workings, and assesses their future.

Trees: Their Natural History, by Peter Thomas (for educators) This reference book provides information on all aspects of tree biology and ecology in easy-to-read and concise language.

This article was written by Lisa Duchene. Lisa is an environmental writer and volunteer coordinator of the Bellefonte Community Children’s Garden in Bellefonte, Pennsylvania.